Bladder tumors were observed in 100% of male p53-deficient (+/-) mice after 24 weeks exposure to 0.5% p-cresidine (5-methoxy-2-methylaniline) in the diet, compared to 75 to 90 weeks in B6C3F1 mice in a two year bioassay. Immunohistochemical analysis for p53 expression was negative and analysis of the genomic tumor DNA indicated that approximately only one out of 5 transitional cell carcinomas show loss of the remaining wildtype allele. No mutations (cold exon-specific single strand conformational polymorphism analysis), which correlate very well with sequence identified mutations, were detectable. Thus, inactivation of one allele, resulting in reduction in p53 gene dosage, may be sufficient to enhance tumorigenesis through genomic instability during genotoxic stress in p53-deficient mice, but the mechanism remains unclear. Induction of mutations or LOH in other critical genes involved in bladder carcinogenesis may also be involved. In addition, we have also observed the induction of liver with a dramatic decrease in latency, which suggests that p53 deficiency may play at least an indirect role in hepatocarcinogenesis in these mice. At the present time we are examining both DNA and RNA from bladder tumors as well as fixed bladder tumors using in situ PCR in situ hybridization (PCR ISH) for p53 and other selected gene(s) for developing insight into the potential molecular mechanisms. To date we have focused on the characterization and evaluation of p53-deficient mice for the identification of chemical carcinogens. Sufficient data have been generated to justify their use in short term carcinogenesis studies for screening chemicals for their carcinogenic potential in mice. The dramatic reduction in latency make this an alternative model for dose metric studies of chemical carcinogenesis. Future studies will address the potential molecular mechanisms for the increased sensitivity of p53-deficient mice to chemical carcinogens.